Tag: maths

Here I’ll exhibit a method from approximating another well known irrational. The Golden Ratio, often denoted , has fascinated people for many years, some going as far as to attribute mystical connotations to it.

crops up a lot in nature and it’s always a pleasure to see it. It’s most easily understood as the positive root of the quadratic , making it equal to . Note that this means it has the property . This time we’ll construct a sequence converging to using the function .

Let , and for all let . Then the first few numbers in this sequence are

One might begin to suspect at this point that , where is the Fibonacci sequence (with ). We can prove this is the case by induction, we’ve already shown the base case by listing the first few values above. Then if we assume that for some , then

.

This sequence is known to converge to , but I’ve never explicitly convinced myself of this before so I’ll do that now. First we’ll define a new sequence by . In doing so we’ve ditched the first element of the sequence and so now for all , . This is because for all we have that implies , which you can check out for yourself. Now we note that for all we have

,

since achieves a maximum of on the square .

(This shows that is a contraction mapping on the interval . At this point we could appeal to the Banach fixed-point theorem and we’d be done. Instead, we directly prove convergence of our series using some inspiration from that theorem).

The next thing we can so is derive the following inequality (from here on in we’re assuming that all numbers we consider are between 1.5 and 2):

Clearly

.

Now if we assume that for some , then

.

Thus by induction for all .

Now we show this sequence is Cauchy convergent by noting that for all

which clearly approaches as and go to . So (and thus ) is Cauchy convergent, and thus convergent since is complete. To see that it converges to , we’ll use the continuity of :

Since the equation has a unique solution for , it must be that .

Thus by iterated application of the function we may approximate . Continued iteration yields ever more accurate approximations.

Let me know if I’ve made any mistakes, I haven’t given myself much proof-reading time before publishing (=

Trigger Warning: This post takes the position that Santa Claus doesn’t exist.

I heard this one from a friend of a friend, it’s a good one. It goes like this:

Consider the logical connective associated with implication, the material conditional as it is sometimes called. It basically captures the intuition behind the “if blah is true then bleep is true”. So if and are two truth values (either true or false) and we write the material conditional as “if then “, then we can write out the four possible outputs:

“if false then false” is true (both are false, implication still makes sense).

“if false then true” is true (remember implies , but can be true whether or not is).

“if true then false” is false ( implies , so this combination is not allowed).

“if true then true” is true ( forces to be true).

The material conditional evaluates to either true or false, depending on whether the truth values are consistent with the implication. Now we can consider to represent the proposition “God exists”. Let stand for “Santa Claus exists”. We can now ask ourselves, is true? Well, clearly is false. What about the implication “if then “? I think everyone would agree that the existence of God in no way implies the existence of Santa, so we can say “if God exists then Santa exists” is false. We now check out the above list and observe that case 3 uniquely matches our situation; the whole implication is false and is false. From this we conclude that must be true, and therefore God exists.
QED

Here’s a neat substitution for integration that I never learned from calculus subjects but found in a book. I also frequently forget the formulae so whenever I want to use it I need to re-derive everything, hopefully writing it here will help me remember.

If is our integration variable, then we make the substitution . Using the trig identities, we can now get expressions for trigonometric functions in terms of .

.

And of course so .
Now we can work out things like , which is given by

I didn’t create this blog until after Pi day, so unfortunately I’m a bit late. I celebrate Pi day by deriving the same series expression for pi every year. I’m not sure who to attribute this particular method to, but I definitely learned in from some old calculus book once upon a time. It’s as simple as calculating two different ways and equating the results. First up, we can make the substitution and simplify yielding . On the other hand, if then converges (uniformly on all intervals ) to , and so may proceed with

We seek closed form for the power series , where , and . First we note that the ratio test yields

, where is the golden ratio. Thus the series converges on the region of the complex plane where . Now let the power series be denoted by , and note that

and so . It seems so unlikely, before you know better at least, that things like that can be done. Why should it be that the nth Fibonacci number is given by , the integral along some closed contour in the complex plane of a function that otherwise seems unrelated. That’s maths!

Once upon a time I was confused about what really made a holomorphic function different from a function on which is differentiable in the real sense. I mean obviously I knew how they were different but I wanted to put it in a context that was more meaningful for me, and after some thinking I thought the following and was satisfied:

Let’s cover some definitions; it’s likely I’ll write these classic definitions in more detail in later posts. Let be a complex function and let be a complex number. is called (complex) differentiable at if the limit

exists, where the limit is taken along whatever path you like. The function is differentiable on a subset if it is differentiable for all . Now a complex number can be written as , where , and a complex function can be written as where and are both functions from to . As -vector spaces, and are isomorphic and we can think of as a function given by

.

Now we it can be said that a complex function is complex differentiable if and only if it is differentiable in the real sense (i.e. all partial derivatives of and exist and are continuous) and and satisfy the Cauchy-Riemann equations ( and ).

This observation is well known and does a good job of contrasting the two senses of differentiability, but I wanted to know the answer to “why is the derivative of the real version of the function a linear transformation (given by the Jacobian matrix) but the derivative of the complex version another complex function?” The answer to that is obvious in hindsight, but I was happy when I went from not knowing to knowing so I thought I better recount it here. The trick is, of course, that we construct a field isomorphism (which I’ll call ) from into (the ring of 2 by 2 matrices with real entries) given by

.

Now taking the derivative of gives us a function , but the Cauchy-Riemann equations mean that if is complex differentiable, then . So in that case we can think of as a function from to , and so my question is resolved! The real version of the function is complex differentiable precisely when the Jacobian matrix can be identified with a complex function through the isomorphism .

The study of groups cleanses the soul. This post will define the concept of a group and show a few simple results, after this there will be several different directions to go in for future posts. Familiarity with sets and functions is assumed.

Definition of Group: A group is really a package of two things. It is a set, which we will denote by , and a function mapping from to . For now we shall denote this function by and call it the multiplication function. The set and the function are together required to satisfy the so called Group axioms:

Associativity: For all , we have that .

Identity: There exists an element of called the identity, which we will usually but not always denote by , with the property that for all .

Inverse: For each element , there exists another element called the inverse of , denoted as , which satisfies .

Sometimes one will encounter abelian groups. An abelian group satisfies the additional property that for all .

The first question to ask is, do any groups actually exist? Our first example of a group will be the infamous trivial group. Let our set contain only one element. The group axioms force us to choose this sole element to be the identity, i.e. . Since there is only once choice for , namely the function that maps to . With these definitions the trivial group satisfies the group axioms.

A more complicated but still basic example is the unique group with two elements. Let be the set and let be given by and . In this case plays the role of the identity element.

In general groups of all cardinalities exist, and our first basic result about groups is that in any group, there can exist only one element satisfying the properties of the identity. To show this, let be a group with identity and suppose there exists an element also fulfilling the role of the identity element. By the properties of we have that . Since also satisfies these same properties, it must be that and thus .

In practice, rather than explicitly referring to the function it is more common to simply write application of as multiplication. For example, rather than we simply write and use parentheses to indicate the order of application. Using this notation we can rewrite the axiom of associativity as requiring that . This is a lot neater, though being explicitly aware of the function is helpful for appreciating the non-triviality of associativity. It will also be useful to refer to the function explicitly when dealing with more than one group, each having their own multiplication functions.

From the definition of a group we can go in several directions. There will be posts delving further into group theory itself, but this post will also serve as a foundation for studying more complex algebraic objects such as rings and fields. Stay tuned.